The present invention relates to solar cell arrays and a means of interconnecting individual cells of the solar cell array and packaging the same.
Solar cell arrays are widely used in space as the primary power source for spacecraft due to their reliability and light weight. Solar cell arrays are also useful in terrestrial applications as an alternative energy source to conventional fossil fuels, nuclear power, or hydroelectric power. When used in space applications, solar cell arrays are normally evaluated based upon three criteria: (1) efficiency of converting solar flux to electrical power; (2) mass of the solar cell array; and, (3) resistance to the space environment.
The general utility of most previously available solar cell arrays has been impaired by the complexity of the known fabrication techniques, attendant excessive costs in terms of time and materials, and low yields of production. Prior methods of interconnecting individual solar cells and packaging the solar cell array have provided operable results. However, with the increasing presence of solar-based power systems, and the decreasing cost of other energy sources, the efficiency of solar cell arrays must be maximized in order for the solar cell arrays to continue as a viable power source. One way to maximize efficiency is to pack the individual solar cells as closely together as possible. Unfortunately, the complexity of interconnecting solar cells has mitigated against maximizing the packing of the individual solar cells.
Also, the prior methods of fabricating interconnections between individual solar cells making up a solar cell array have provided less than satisfactory results with regard to protecting the solar cells and interconnect circuitry from degradation due to exposure to the space enviroment. Although it is well known that arrays of solar cells can be mounted on flexible substrates for use aboard satellites and the like, many of the previous solar cell arrays have included electrical circuitry that is deposited on top of or below the flexible substrate where it is directly exposed to the space environment. Because of the delicate nature of the flexible substrate, it is susceptible to erosion by atomic oxygen bombardment. Some of the materials used to fabricate the solar cells are also susceptible to erosion by atomic oxygen bombardment. Also, the direct exposure of the circuitry to the ambient plasma environment in the neighborhood of the spacecraft causes leakage currents to flow when high voltages are generated by the array.
One attempt to isolate the circuitry from direct exposure to the space environment is reported in U.S. Pat. No. 4,133,697 to Mueller, et al. The '697 patent isolates the electrical circuitry from the environment by sandwiching the circuitry between a pair of layers of a polyimide material, wherein at least one of the layers has a plurality of apertures containing solder pads. The solar cells are mounted on top of the layer including the apertures and an electrical connection between the solar cells and the circuitry results when the solder is heated to an elevated temperature and flows within the apertures. The solar cells used in the '697 patent are characterized by wrap-around electrical contacts that provide an electrical connection between the top surface of the solar cell and the bottom surface of the solar cell. However, the use of wrap-around contacts leads to an undesirable increase in the complexity of the fabrication of the individual solar cells and undesirable constraints on the design of the cell and array configuration.
Lightweight, high efficiency, and radiation resistant photovoltaic cells have been developed in order to meet the high specific power (watts/kilogram) requirements of spacecraft. One way of designing photovoltaic cells that have high specific powers is to reduce the weight of the semiconductor materials. One means of reducing the weight of a semiconductor is to reduce its overall thickness. Since most components of solar cells (including semiconductors, substrates and coverglasses) are relatively brittle, especially when made thin, decreasing the thickness of these semiconductors results in an increase in their fragility. The fragility of the semiconductors and photovoltaic cells that include such semiconductors becomes readily apparent during conventional fabrication of solar arrays when as many as 25 to 30 percent of the photovoltaic cells are damaged or broken.
Another way to improve the specific power of solar cells is to increase the conversion efficiency without weight penalty. Tandem cells can achieve this increase in conversion efficiency, be they mechanically stacked tandem cells or monolithically integrated multijunction photovoltaic cells. The mechanically stacked tandem cells include at least two subcells stacked one on top of the other and held together by an adhesive or other bonding techniques. The monolithically integrated multijunction cells include various semiconductor layers deposited directly on top of each other. The mechanically stacked tandem photovoltaic cells can be produced in yields that are greater than the yields obtained during the production of monolithically integrated multijunction photovoltaic cells. This greater yield is the result of the ability to (1) increase the process latitude of building subcells because the subcells are built independently of each other and (2) to select individual subcells for the mechanically stacked tandem cell that are optimally matched, in contrast, when producing the monolithically integrated multijunction photovoltaic cells, damage to, or poor performance of one of the subcells requires that the entire cell be replaced because the subcells cannot be separated without irreversible damage. Prior configurations of arrays for photovoltaic cells, particularly mechanically stacked tandem photovoltaic cells, have not provided the flexibility of electrical interconnection that is desired in order to efficiently design and optimize the performance of the array. Prior arrays for photovoltaic cells have also been less than satisfactory in their ability to prevent damage to the photovoltaic cells and interconnection schemes during fabrication, handling, deployment, and operation of the array.